If you use SSH frequently to access unix.andrew.cmu.edu, you must have tried at some point to set up passwordless login using public/private keys. That didn't work for me, and when I did ssh -vv unix.andrew.cmu.edu, it always failed at

debug2:wedidnotsendapacket,disablemethod

and then asked me for a password. After much trial and error (including locking myself out of my account), I finally uncovered the secret.

Note: this tutorial is for Mac OS X only, although you might be able to
configure a similar setup on Linux.

Setup A Few Things

First, let's get a basic things out of the way. These are optional, but I will assume that you have these for the rest of the tutorial.